The present invention relates generally to light emitting diodes. More specifically, the present invention relates to methods and apparatus for performing wavelength-conversion using phosphors with light emitting diodes.
Until recently, the external efficiency of light-emitted diodes (LEDs) that emitted light having wavelengths between ultraviolet and blue was very low, e.g., in the range of thousandths of one percent, relative to other devices such as red-emitting AlGaAs LEDs. Achievements in InGaN/AlGaN quantum well devices, however, have allowed the construction of LEDs emitting wavelengths between ultraviolet and blue with efficiencies comparable to those of the best red-emitting or yellow-emitting LEDs. For example, the external efficiency of InGaN/AlGaN LEDs at room temperature are reported to reach 10% at emission wavelengths of 400 to 450 nm. See Nakamura et al., Appl. Phys. Lett. 67 (13), 1995, p. 1868; see also U.S. Pat. No. 5,959,307 to Nakamura et al. which is incorportated herein. Taking into account the high refractive index of InGaN/AlGaN, a 10% external efficiency implies an internal efficiency close to 100%. Thus, the internal efficiency of these LEDs emitting wavelengths between ultraviolet and blue is higher than that of well known red-emitting or yellow-emitting LEDs.
Due to the relatively high efficiencies of LEDs emitting wavelengths between ultraviolet and blue, such devices can form a superior basis for designing devices that emit light having a wavelength(s) from green to red. Known systems, however, fail to incorporate LEDs emitting wavelengths between ultraviolet and blue effectively.
An apparatus, comprises an active region, a phosphor layer and a reflective layer. The active region is configured to emit light having a first band of wavelengths from a first group of wavelengths. The phosphor layer is disposed between and in contact with the active region and an exterior medium. The phosphor layer is configured to convert the first band of wavelengths of light emitted from the active region to a second band of wavelengths. A center wavelength of the second band of wavelengths is greater than a center wavelength of the first band of wavelengths. The reflective layer is optically coupled to the active region. The active region is disposed between the reflective layer and the phosphor layer. The reflective layer is configured to reflect at least the first band of wavelengths and the second band of wavelengths.